What do you do when your ancestors really are named John and Mary? Hope for children with unusual names? There are certain strategies that can help locate even those with common given and surnames. I would also start by giving my standard advice, find out everything you can about the next generation closer in time before you take the plunge into the morass of a common name. If I put a number of pencil dots on a piece of paper, there is apparently nothing to differentiate the dots from each other. They all look exactly the same. Except for one very important fact, none of the dots are in exactly the same location. Even individuals with identical names, even individuals that have identical names with identical names of their spouses, and the same children's names in the same order, lived in exactly the same place and time.
My dots on the paper can easily be identified by overlaying the paper with a grid and identifying each dot by its location in the grid. Likewise, identifying geographic locations, even down to the name of the farm or house, might be necessary to identify an individual with a common given name and surname. So, you say, easier said than done. Yes, you are right, it is easier said than done. But it is one of the only ways to distinguish between those individuals with common surnames. Another way of distinguishing between individuals is to look at the pattern of the family. Even two families with the same names for the parents and each of the children will not likely have the children in exactly the same order (although it has been know to happen). In this extreme case, you have to focus even more sharply at slight differences in location and the ages of the children. This is a technique commonly used in Danish/Norwegian research because of the paucity of given names.
You may also gain some ground by looking at other distinguishing characteristics of the person, such as, occupations, land ownership, church affiliation, and other specific characteristics. But where do you find these specifics? Sometimes they are right before your eyes in the Census records, sometimes, it may take years of searching local records such as newspapers and court records to come up with two or even one difference that divides the families into separate groups.
In our instant gratification society, we don't want to postpone our reward indefinitely, so we quit. Don't quit. Don't bang your head against the wall, but don't quit. Keep looking for more records from the same location where the ancestor was definitely identified. If you don't have a definite place or date for the parents, concentrate on the first child you can positively identify. Stop trying to find the parent until you know the whole life story of the child.
Look for the small clue, I once found a family by looking for a "relation" named in a will probate. The family had been looking for years and had not picked up on this clue, apparently. By looking in the general geographic area of the will probate, I found almost two dozen family members, some of which had the same names as those of the ancestor's family. This technique is a lot more productive than searching census records for people with the same name, believe me.